Hydropyrimidines. V. Hexahydropyrimidines. The reaction of aldehydes and ketones with 1,3-diaminopropanes

Abstract

Hexahydropyrimidine and some N-alkylated derivatives have been obtained from the reaction of the corresponding trimethylenediamine or its monoprotonated salt with formaldehyde. A variety of spectroscopic evidence supports a cyclic structure for these compounds in preference to a tautomeric open-chain form. The chemical behaviour of hexahydropyrimidine is explicable in terms of a cyclic di-secondary amine structure. The cyclic structure is destabilized with respect to the open- chain form if the hydrogen atoms attached to C2 of the hexahydropyrimidine ring are replaced by alkyl groups and cannot be detected when, in addition, one of the hydrogen atoms attached to nitrogen is replaced by the bulkier t-butyl group. Hexahydro-2-methylpyrimidine is dehydrogenated to 1,4,5,6- tetrahydro-2-methylpyrimidine on shaking with Adams catalyst and hydrogen under laboratory conditions. Hexahydropyrimidines which are tautomeric mixtures are reduced to the corresponding N-alkyl-1,3- diaminopropanes, while those possessing cyclic structures are inert.